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[HCG24-P08] Genesis of Tsukinuno bentonite deposit indicated from the mineralogical and geochemical characteristics
Geological disposal facilities for radioactive wastes are envisaged to use bentonite as engineered barriers, due primarily to its swelling property, high cation exchange capacity, and low permeability. Longevity of the bentonite in geological timescale is essential to secure the performance of bentonite. One of possible alteration processes for the bentonite is cementation of clay layers by the precipitation of secondary minerals such as silica. Whereas a previous study[1] has tested the physical properties of bentonite ores, which have been subjected to cementation and compaction during diagenesis, formation pathway and conditions of the bentonite ores need to be constrained to make such a study to useful natural analogues for cementation of bentonite in radioactive waste disposal facilities. In this context, we investigated the genesis of Tsukinuno bentonite deposit, northeast Japan, to clarify its availability as a natural analogue site.
Tsukinuno bentonite deposit, containing 31 bentonite beds, has been thought to form by the diagenesis of volcanic ash ~10 Ma. The compositions of trace and immobile elements (Nb/Y vs Zr/TiO2) indicated that most of the bentonite beds were derived from rhyolitic-dacitic volcanic ash. No correlation was observed between the estimated chemical compositions of original ash and the amounts of montmorillonite in the ores, suggesting that the current mineral composition of each bed varies, likely depending on the supply of Mg2+ from seawater after sedimentation. Clinoptilolite and analcime were the zeolite phases observed as accessory minerals in the upper and lower beds, respectively. This difference suggests a steep geothermal gradient in the deposit, in which the lower beds were likely to have experienced a higher temperature. This is also consistent with beidellitization of the lower beds, estimated by Greene-Kelly test on XRD profiles. Our findings suggest that bentonite ores from Tsukinuno bentonite deposits could be used to understand temperature effects upon cementation of bentonite buffer in the disposal facilities.
Reference
[1]JAEA (Japan Atomic Energy Agency)・RWMC (Radioactive Waste Management Funding and Research Center) (2019) The project for validating near-field assessment methodology in geological disposal, H30 Annual Report (in Japanese).
Tsukinuno bentonite deposit, containing 31 bentonite beds, has been thought to form by the diagenesis of volcanic ash ~10 Ma. The compositions of trace and immobile elements (Nb/Y vs Zr/TiO2) indicated that most of the bentonite beds were derived from rhyolitic-dacitic volcanic ash. No correlation was observed between the estimated chemical compositions of original ash and the amounts of montmorillonite in the ores, suggesting that the current mineral composition of each bed varies, likely depending on the supply of Mg2+ from seawater after sedimentation. Clinoptilolite and analcime were the zeolite phases observed as accessory minerals in the upper and lower beds, respectively. This difference suggests a steep geothermal gradient in the deposit, in which the lower beds were likely to have experienced a higher temperature. This is also consistent with beidellitization of the lower beds, estimated by Greene-Kelly test on XRD profiles. Our findings suggest that bentonite ores from Tsukinuno bentonite deposits could be used to understand temperature effects upon cementation of bentonite buffer in the disposal facilities.
Reference
[1]JAEA (Japan Atomic Energy Agency)・RWMC (Radioactive Waste Management Funding and Research Center) (2019) The project for validating near-field assessment methodology in geological disposal, H30 Annual Report (in Japanese).